Load isolation network which protects transmitter if antenna is disconnected



LOAD ISOLATION NETWORK WHICH PROTECTS TRANSMITTER IF ANTENNA IS DISCONNECTED Filed D60. 12, 1966 1' TRANSMITTER DRIVER STAGE lnvenior WILLIAM A SCHILB United States Patent I 3,424,983 LOAD ISOLATION NETWORK WHICH PROTECTS TRANSMITTER IF ANTENNA IS DISCONNECTED William A. Schilb, Lombard, Ill., assignor to Motorola, Inc., Franklin Park, Ill., a corporation of Illinois Filed Dec. 12, 1966, Ser. No. 601,140 U.S. Cl. 325-150 Int. Cl. H04b 1/02 3 Claims ABSTRACT OF THE DISCLOSURE In order to prevent damage to the power output transistors of communication devices it is necessary to provide protective circuits for them. This is particularly true in miniature transmitters where the transistors in the power output stages are operated as close to their maximum ratings as possible. The power output transistors are coupled to an antenna which is designed to provide a matched load so that all of the power from the power output transistors reaching the antenna is absorbed by the antenna.

In using the communication devices the impedance of the antenna load may be changed due to mistuning or to a malfunction of the antenna. For example the antenna connection may become loose or the antenna may break off or strike a tree or a bush. All of these actions will change the impedance of the antenna so that it becomes a reactive load instead of a resistive load. Under these conditions power is reflected from the antenna back to the power output transistors adding to the power applied to the transistors. This increase in power is often enough to damage or destroy the transistors before normal protective circuits can act to limit this damage.

It is therefore an object of this invention to provide a load isolation network for coupling power from output transistors to an antenna and to protect the output transistors from damage caused by power reflected from the antenna.

A feature of this invention is the provision of a matching network including a transformer having a pair of windings inductively coupling the power output transistors to the antenna. Each of the pair of windings forms a portion of a separate parallel resonant circuit tuned to the transmitter frequency. A third resonant circuit is inductively coupled to the pair of parallel resonant circuits through a third winding of the transformer and directly connected to each of the pair of tuned circuits.

The invention is illustrated in the single drawing, a partial schematic and a partial block diagram of the output stages of a transmitter incorporating the protective features of this invention.

In practicing this invention a transformer is provided having first, second and third windings. Each of the Windings forms a portion of first, second and third resonant circuits tuned to the frequency of the transmitter. The first and second windings inductively couple the output transistors to the antenna. The third resonant circuit also includes a resistor and is connected to the first and sec- 0nd resonant circuits. In normal operation power is coupled from the power output transistors to the antenna. If the load presented by the antenna becomes reactive the power coupled to the antenna is reduced and the excess power is coupled to the third resonant circuit where it is dissipated by the resistor.

An embodiment of the invention is shown in the drawing. A transmitter portion 16 supplies radio frequency signals, which may be modulated or unmodulated, to driver stage 18. Driver stage 18 amplifies the signals and they are applied to base 21 of power output transistor 20. The output of transistor 20 is taken from collector 22 and coupled to filter stage 28 through capacitor 24 inductance 25 and capacitor 26. Filter 28 is coupled to antenna 41 through the load isolation network.

The load isolation network includes a transformer 35 having three windings, an input winding 33 and two output windings 40 and 45. The inductance of winding 33 together with the parallel capacitance of capacitor 32 form a tuned circuit 30 which is a resonant at the frequency of the signal from transistor 20. Transformer winding 40 and capacitor 39 form an output tuned circuit 37 resonant at the same frequency as tuned circuit 30. Tuned circuit 37 is coupled to antenna 41. A third tuned circuit 42 consisting of transformer winding 45 connected in parallel with capacitor 44 and resistance 48 is provided. Tuned circuit 42 is also resonant at the frequency of the signal transistor 20. One end of parallel resonant circuit 42 is connected to parallel resonant circuit 37 and the other end of parallel resonant circuit 42 is coupled to parallel resonant circuit 30 through capacitor 46. Resistance 48 is very much greater than the source impedance driving the load isolation network and may be of the order of 10 times this source impedance.

In normal operation signals from filter 28 are inductively coupled from transformer winding 33 to transformer winding 40 and from there to antenna 41. If the impedance presented by antenna 41 changes so that energy reflections can occur, the circuit is unbalanced so that the energy coupled from tuned circuit 30 to tuned circuit 37 is reduced and the energy coupledh t'o tuned circuit 42 is increased. The energy coupled to tuned circuit 42 flows through resistor 48 and is dissipated so that energy is not reflected back to transistor 20. When the load presented by antenna 41 is again at the proper impedance the energy coupled to tuned circuit 37 increases and the energy coupled to tuned circuit 42 decreases.

The insertion loss caused by the load isolation network may be reduced by phasing the windings as shown in the drawings. The signals across output winding 40 are in phase with signals across input winding 33 while the signals across output winding 45 are out of phase with the signals across input winding 33.

I claim:

1. A load isolation matching network for coupling a transistor output stage of a transmitter having a predetermined impedance to an antenna for applying a signal having a particular frequency thereto, including in combination, transformer means, a first parallel resonant circuit resonant at said particular frequency coupled to the transistor output stage and including said transformer means, a second parallel resonant circuit resonant at said particular frequency coupled to the antenna and including said transformer means, said transformer means acting to couple inductively said first and second resonant circuits, a third parallel resonant circuit resonant at the particular frequency and including said transformer means, resistance means coupled in parallel with said third parallel resonant circuit, said resistance means having an impedance very much greater than the predetermined impedance of the transistor output stage, and capacitance means coupling said first parallel resonant circuit to said third parallel resonant circuit.

2. The load isolation network of claim 1 wherein, said transformer means includes a first winding forming a portion of said first parallel resonant circuit, a second winding forming a portion of said second parallel resonant circuit, and a third winding forming a portion of said third parallel resonant circuit.

3. The load isolation network of claim 2 wherein, said first parallel resonant circuit includes a first capacitor connected in parallel with said first winding and coupled to the output transistor stage, said second parallel resonant circuit includes a second capacitor connected in parallel with said second winding and coupled to said antenna, and said third parallel circuit includes a third capacitor and a resistor each connected in parallel with said third winding, circuit means connecting said third References Cited UNITED STATES PATENTS 2,550,486 4/1951 Loughlin 334- 63 X 2,854,665 9/1958 Brown l 343-180 3,286,201 11/1966 Roberte 333-8 X HERMAN KARL SAALBACH, Primary Examiner. PAUL L. GENSLER, Assistant Examiner. I

s. or X.R. 32s 17s; 333-24; 343 s5o 

